NMR spectroscopy is a powerful analysis method. HF (=high-frequency) pulses are irradiated into a measurement sample, which is in a strong static magnetic field, whereby nuclear spin is aligned in the measurement substance, and the HF reaction of the measurement sample is measured. Information is thus obtained integrally over a certain region of the measurement sample, known as the active volume, and is analyzed in order to determine the chemical composition of the measurement sample.
In order to reduce line broadening on account of anisotropic interactions, it is known to allow an NMR sample to rotate at high frequency (typically several kHz) during the spectroscopic measurement, tilted at what is known as the “magic angle” of arctan√2≈54.70° relative to the static magnetic field. To this end, the sample is filled into a MAS rotor. MAS rotors are generally cylindrical tubes open on one side, which are closed by a cap, wherein the cap is provided with wing elements. The MAS rotor is arranged in a MAS stator, and the MAS rotor is driven with gas pressure via the wing elements for the rotation.
Document EP 3 093 679 B1 (=reference [2]) describes an NMR-MAS probe head having all physical components of the generic probe head defined at the outset, which probe head however—contrary to the teaching of reference [1]—is not pivotable. The loading and unloading of the MAS stator with a MAS rotor containing a new NMR sample before and after each NMR measurement is thus relatively complicated each time, since the stator cannot be brought into an optimized loading position for the insertion of the rotor with the receiving opening generally arranged parallel to the z-axis of the NMR magnet system, but instead always remains in a position inclined relative thereto.
Document DE 10 2014 201 076 B3 (=reference [3]) also describes an NMR-MAS probe head in which the MAS stator likewise is formed non-pivotably. In addition, reference [3] however also discloses an embodiment of this probe head in which an additional device is provided which makes it possible to identify from outside whether a transport container is currently fitted with an NMR-MAS rotor. To this end, an additional mechanical component is moved by the MAS rotor inserted into the transport container in such a way that it is possible to identify from outside on the transport container, visually or with sensors, whether or not the transport container is fitted with a MAS rotor.
In U.S. Pat. No. 7,915,893 B2 (=reference [4]) an NMR-MAS probe head for a CryoMAS probe is disclosed, in which the MAS stator is again arranged non-pivotably in the probe head, but a contactless optical measurement of the rotational frequency of the MAS rotor is possible in the operating state.
In contrast to the three references [2] to [4] discussed above, the document DE 10 2013 201 110 B3 (=reference [1]) cited at the outset discloses an NMR-MAS probe head—generic in respect of the present invention—in which the MAS stator is mounted pivotably in the probe head in order to reduce the curve of the movement of the MAS rotor as it is inserted into the MAS stator, and vice versa. For transfer, the MAS stator should then be pivoted about the magnetic center. This probe head should thus be able to be made even more compact.
This generic probe head with the features defined at the outset still has the following disruptive disadvantages, however:
On the one hand it is not possible to mount a mechanical device for identifying the loading state of the MAS stator, as is proposed in reference [3]. Specifically, this device presumably would not survive a pivoting of the MAS stator over a relatively large angle between a measuring position and a loading position—as proposed in reference [1]—without sustaining damage, and certainly not after multiple pivoting operations.
The—desirable—pivotability of the MAS stator, however, also conflicts with a frequency detection of the MAS rotor rotating in the operating state during an NMR measurement, as proposed in reference [4]. A stationary light guide would inevitably break at its MAS stator-side end—at least after a number of pivoting operations of the MAS stator. The same would presumably also occur with use of a wire instead of the light guide for an electrical, instead of optical, frequency detection.
A loading recognition according to reference [3] in which, however, an electrical feed or an optical light guide according to reference [4] would be used for the MAS stator instead of the mechanical device proposed in reference [3] would also only be possible with a non-pivotable MAS stator for the reasons cited above, because otherwise a high risk of breakage of the guide elements on the moved MAS stator would have to be reckoned with.
It is nevertheless desirable to provide an at least optical detection device for MAS probe heads with pivotable MAS stator which do not have these disadvantages, such that for example a—usually relatively brittle—light guide is not exposed to any threatening mechanical stressing and therefore is not at high risk of breakage, even during continuous operation.